In recent years, to cope with automobile exhaust gas regulations which have been reinforced year by year, a honeycomb structure (catalyst body) supporting a catalyst has been used in order to remove nitrogen oxide, sulfur oxide, hydrogen chloride, hydrogen carbide, carbon monoxide and the like contained in an automobile exhaust gas. The catalyst supported by partition walls of the honeycomb structure adsorbs and decomposes the above-described toxic substances contained in the automobile exhaust gas to purify the automobile exhaust gas.
Since a catalytic activity of the catalyst for use in the honeycomb structure (catalyst body) supporting the catalyst usually increases in a high-temperature region, an automobile is run in a low state of the catalytic activity, and an insufficiently purified exhaust gas is exhausted from start of the running of the automobile until a temperature of the honeycomb structure rises. Therefore, the operation of the honeycomb structure at the low temperature needs to be performed in a time as short as possible. As one of methods, a thermal capacity of the honeycomb structure is reduced, and the temperature of the honeycomb structure is raised in a short time after the start of the running of the automobile. To reduce the thermal capacity of the honeycomb structure, a weight needs to be reduced (bulk density needs to be reduced) without changing a geometric surface area of the honeycomb structure, and to attain this, there is a method in which a thickness of the partition wall of a cell is reduced or a porosity is increased. However, the reduced bulk density by the thinning of the partition wall of the cell and the increasing of the porosity has caused a drop in mechanical strength of the honeycomb structure. A catalyst body for purifying a large flow rate of exhaust gas exhausted from a large-sized automobile such as a truck also requires a large volume, and a honeycomb structure which is a catalyst carrier having a large section is required to lower a pressure loss. As described also in Japanese Patent Application Laid-Open No. 3-275309, there has been a problem that a cell partition wall in an outer peripheral part of the honeycomb structure is deformed under the weight of the honeycomb structure at the time of extrusion.
To prevent the mechanical strength from dropping, it has been proposed that after forming and firing the honeycomb structure, a partition wall deformed region of an outer peripheral part of the structure should be removed to coat an outer peripheral surface of the structure with an outer wall by ceramic cement coating (form an outer shell layer) (Japanese Patent Application Laid-Open No. 5-269388). However, in this method, the mechanical strength of the honeycomb structure can be increased, but the thermal capacity of the outer wall increases, and heat inside the honeycomb structure (cell structural part) escapes toward the outer wall side (expands). Therefore, there has been a problem that a temperature rising rate of the honeycomb structure at the start of the operation drops and that the catalytic activity of the supported catalyst does not easily increase in a short time. As described in Japanese Utility Model Application Laid-Open No. 63-144836, it has been proposed that a coating layer should be disposed on the outer peripheral surface of the integrally extruded honeycomb structure to improve an outer diameter precision of the honeycomb structure. When the partition wall is thinned, the mechanical strength of the honeycomb structure drops. However, when the mechanical strength drops to such an extent that the structure cannot resist canning bearing, a method is used in which the outer diameter precision of the honeycomb structure is improved to set a clearance range at a canning time to be appropriate in order to reduce the canning bearing. However, in this means for disposing the coating layer on the outer peripheral surface of the integrally manufactured honeycomb structure, resistance to the canning of the honeycomb structure can be improved, but the same problem as that of the above-described honeycomb structure including the outer periphery coated with ceramic cement occurs that the thermal capacity of the outer wall increases and that the heat inside the honeycomb structure (cell structural part) escapes toward the outer wall side (expands).
The present invention has been developed in consideration of the above-described problem, and an object thereof is to provide a honeycomb structure, for example, in which an outer peripheral region of the integrally formed honeycomb structure is removed, and an outer wall by ceramic cement or the like is disposed on an outermost side of the honeycomb structure (on the outer peripheral surface of a cell structural part) and in which a temperature rising time for the cell structural part after start of an operation shortens and in which a catalytic activity can be increased in a short time when a catalyst is supported, a manufacturing method capable of efficiently manufacturing the honeycomb structure, and a catalyst body in which a catalyst is supported on the honeycomb structure. The present invention fulfills an effect especially in a honeycomb structure whose outer peripheral region is removed after the honeycomb structure is integrally formed and whose outer wall needs to be newly formed by a coating by ceramic cement or the like, and is especially effective for a honeycomb structure including a thin partition wall or having a large sectional-area. For example, in a cordierite honeycomb structure having a diameter of around φ100 mm for use in a catalyst body for purifying an exhaust gas from an automobile, in recent years, the structure having a reduced partition wall thickness of 6 mil (0.152 mm) or less has been put to practical use, and the present invention is especially effective in a thickness of 3 mil (0.076 mm) or less. A cordierite honeycomb structure including a large section having a diameter of approximately φ130 mm or more for use in a catalyst body or a filter for purifying an exhaust gas from a large-sized diesel car has been put to practical use, the partition wall usually having a thickness of 6 mil or more has been generally used, but the present invention is effective also in the honeycomb structure, or further effective especially in the honeycomb structure having a partition wall thickness of 6 mil or less. A cell density for general use is 300 to 1200 cpsi (cells/square inch) in the honeycomb structure for the catalyst body, or 100 to 300 cpsi in the honeycomb structure for the filter, but the density is not restricted to them. Furthermore, outer wall forming means is not restricted to only the methods seen in the related arts, and methods such as flame spraying may also be used.